1. Field of the Invention
This invention relates to optical amplifiers which may be used for optical communications and optical information processing, etc. More specifically, the invention relates to semiconductor optical amplifiers in which gain may be controlled.
2. Description of Related Art
Optical communication systems have recently attracted considerable interest. The majority of such systems employ electronic circuits. For example, in an optical repeater, an optical signal is converted into an electrical signal and the electrical signal is shaped and processed. Then, the electrical signal is converted back into an optical signal.
In such an optical repeater, the complicated processing causes the operation rate and phase changes, etc., of the electronic elements used in an electronic circuit, to become marginal as the transmission rate of the optical signal becomes faster. Therefore, it is desirable to form a compact and economical optical repeater with a high transmission rate by processing optical signals without conversion into electrical signals. It has also been known to form an optical amplifier to provide input signals to a pin photodiode instead of using a photodetector with gain, such as an avalanche photodetector.
When an optical amplifier is used as an optical repeater or photodetector, it is necessary to control the gain or the output level of the optical amplifier. However, in a known laser amplifier, the gain depends considerably on ambient temperature, wavelength and polarization of the input optical signal. Thus, the laser amplifier possibly oscillates even if the reflection factor from a face of the laser amplifier is restricted to be low. It is indispensable to control the gain of the optical amplifier in an actual system.
FIG. 13 shows a known optical amplifying device in which gain is controlled to ensure a constant output. In the device, current source 90 supplies a bias current to laser amplifier 91 through resistors Ra and Rb and inductance L. Differential amplifier 92 detects the voltage across resistor Ra. Also, amplifier 93 amplifies a signal corresponding to current variations at a point where inductance L and resistor Rb are connected. Multiplier 94 multiplies the signal from amplifier 93 by an output signal from differential amplifier 92. An output signal from multiplier 94 is supplied to filter 95.
Rectifier 96 rectifies the signal from filter 95 to obtain a direct current signal. Differential amplifier 97 amplifies a signal corresponding to the voltage difference between the direct current signal and a reference signal. Also, integrator 98 integrates the signal from differential amplifier 97. The signal from integrator 98 controls the bias current from current source 90 to ensure a constant output optical signal from laser amplifier 91.
However, in this device, oscillations are possible because the gain of laser amplifier 91 itself is not monitored. Therefore, optical amplifiers have not yet been put into practice and electronic circuits have been largely employed for amplification.